Structural fibre has a significant effect on the motor activity of the rumen. This activity depends on physically effective neutral detergent fibre (peNDF) in TMR. The peNDF is determined by multiplying the content of the NDF in DM and the % residue on the 8 and 19 mm sieves. Beauchemin & Yang (2005) confirmed that particle length is a reliable indicator of the rumination period, but it is not necessarily an indicator of rumen acidosis. Acidosis is defined as a state of high pathological acidity of the blood, and the incidence is increasing in ruminants. The duration at which pH remained below the threshold value of 5.8 in a 24-h period (Valente et al., 2017) was used as an indicator to characterize of subacute ruminal acidosis (SARA). Plaizier (2004) determined that a peNDF of 12.5% DM or lower resulted in a rumen pH indicative of SARA. Rumen acidosis could be supposedly reduced by using ruminal mechanical stimulating (RMS) brushes, European patent EP0609045A2 (Meiwa-Sangyo Co. Ltd, Kyoto, Japan), commercially known as Rumenfibe (RF). This device administered as three pieces per dairy cow could be used for stimulating the physical function of the rumen mucosa. Golder et al. (2017) examined the effects of RMS brushes on rumen fermentation and subsequent milk production in early lactation dairy cows from a commercial pasture-based herd in the Australian spring and summer. Consequently, rumen fermentation properties and milk production were not affected by RMS brushes administration. The present paper evaluates the beneficial effect of RMS brushes administration on the performances of high-yielding dairy cows in early lactation fed on TMR with minimal structural fibre, i.e., low peNDF (10.9% DM).